The kidneys regulate the water and salt balance in the body, maintaining hydration, electrolyte levels and regulating blood pressure. As proteins are metabolized by the body for energy, by-products are produced and circulated in the blood. It's the kidney's job to remove these toxic substances. Waste products such as urea nitrogen, creatinine and phosphorus, as well as certain drug metabolites, are all filtered from the blood and excreted in the urine. You may be familiar with the names of these by-products of protein breakdown if your cat has had blood work done. These are what are measured in the blood to detect declining kidney function – high levels mean that the kidneys aren't working normally. A complication is that the kidneys possess an amazing capacity for compensation. As much as 75% of kidney function must be lost before we can detect abnormally high blood values for these substances. Sometimes we see increased thirst and urination before the blood values rise above normal, as the kidneys become less able to conserve water, but not always.